1. Field of the Invention
The present invention relates to a method utilizing an aerodynamic interlacing process to produce a chemical filter media, more particularly to an improved method through which bi-component fibers and functional particulates are incorporated into a composite structure of a chemical filter media.
2. Description of the Prior Art
Functional particulate, such as activated carbon, magnesium bicarbonate, zeolite . . . etc, has long been used in the elimination of chemical contaminants. In applications that require low level of chemical contamination to be eliminated (xe2x89xa650 ppm), these functional particulates are generally loaded onto a fibrous web to meet converting requirements such as pleating or molding.
With respect to the forming of particulate loaded web, several existing inventions have focused on the benefit resulting from the absence of resin adhesives. This is because resin adhesive undesirably block large portions of particulate""s open surface area therefore drastically reduces the media""s ability to adsorb.
This is especially important in low level contamination applications because at low concentration level, contaminants can not penetrate deeply into body of particulate, only outer layer of particulate surface can be effectively utilized for adsorption. Primary filtration mechanism for low pollutant concentration level is diffusion. That is, pollutant is diffused from airstreams into surface of sorbent particulate, hence held within sorbent particulate by Van der Waals force. When pressure gradient is low, diffused pollutant can not penetrate deeply into body of particulate. Therefore maximizing particulate""s outer surface exposure to contamination is essential in prolonging filter service life, which terminates when pollutant penetration reaches a predetermined level.
Many inventions and disclosures are available in the application scopes of resin adhesive free chemical filter media. In U.S. Pat. Nos. 3,971,373 to Braun, a process of immobilizing particulate matter in a melt blown web is described. Although results from a melt blown process has the advantage of very fine pores which provides the filter media an additional mechanical efficiency for particulate filtration. However, its micro fibers and small pore sizes also have the disadvantage of high resistance and low tensile strength characteristics. The high resistance characteristic excludes this type of filter media from applications where higher particulate density is required. The lack of tensile strength also limits this type of filter media to few converting processes.
U.S. Pat. Nos. 5,486,410 to Groeger el, describes a process of loading particulate matters into a preformed fiber matrix. Although this process allows for wider particulate density range, however, in order to achieve higher particulate density using this method, multiple layers must be bonded together. Because this process require particulate matters to deposit into preformed cells in the web, evenness of particulate distribution on the web is constrained by the evenness of pre-constructed cell formation and arrangement. Size of opening space (air channel) formed by adjacent particulate is predetermined by size of preformed cells in the web. In addition, because particulates are likely to be trapped by intersections of bonded fibers, spaces between adjacent particulates, or air channels, are left largely open.
This invention addresses an important issue essential to maximizing particulate""s surface exposure, that is to create a superior microstructure within a particulate loaded web. As particulates load up in a web, spaces, or air channels are formed between adjacent particulates. Larger the air channels, faster and more directly the air can move through the filter media, thus shorter the air residence time in the filter media. The result from this disadvantaged scenario is two fold. First, the direct airflow will exhaust only particulate surface area facing air channels, therefore service life of filter media will terminate when there is still sizable portion of unused particulate surface area. Second, because pollutant is allowed to pass through filter media with higher momentum, the effectiveness of diffusion mechanism reduced.
On the other hand, if sizes of air channels and speed of airflow is reduced, path of airflow is more tortuous, and air residence time is increased, pollutant will be exposed to more particulate surface area before it exits the filter media. In another word, the utilization of filter media""s total particulate surface area is substantially increased, and the chance for sorbent particulates to grab diffused pollutant in the airflow is largely improved.
The process of invention will produce a chemical filter media in form of a sorbent particulate loaded fiber web, which will cause air channels formed by adjacent particulates filled by multiple number of fibers to create a diverting mechanism. This diverting mechanism, will significantly reduce the size of air channels, and will cause pollutant in airflow to travel in a tortuous path therefore result in significantly longer residence time, and will result in higher exposure to surface of sorbent particulates thus result in greater probability to diffuse into sorbet""s surface area. Ultimately, chemical filter media produced using process of invention will result in significantly longer service life than those of equivalent density but produced using different process.
It is the primary objective of the present invention to provide a method and structure for producing chemical filter media. This objective is achieve primarily by an aerodynamic interlacing process and secondly by an innovative thermo process. Short-cut bi-component fibers and functional particulate (e.g. activated carbon, potassium permanganate impregnated aluminum oxide and chemical adsorptive macromolecule) are the basic materials. A consistent flow of air is responsible for engaging fibers and particulate in mixing, interlacing and forming processes to create a sheet of nonwoven chemical filter media.
Another objective of the present invention is based on the aerodynamic interlacing process to create a chemical filter media which does not require the use of a preformed fiber web as substrate. The advantage is, because short cut fibers are used to evenly mix with particulate during the aerodynamic forming process, the evenness and coherence of fiber/particulate integration, the evenness of pore distribution, the structure consistency, are all unsurpassed by other methods that require a preformed fiber web to load up particulates. In addition, the aerodynamic interlacing process allows for fiber/particulate mixing ratio to be controllable over a wide range. The choice of particulate mesh size and fiber denier size can be varied according to the requirement of final media characteristics. Moreover, high particulate density can be achieved in single pass, without the need to laminate several layers of lower density media to achieve high density objective. Therefore, structure consistency and coherence is high.
A further objective of the present invention is based on the aerodynamic process to achieve a controllable gradient structure. By adjusting airflow behaviors in the forming chute, resulting fiber/particulate distribution through the thickness of media may vary. Fiber/particulate may either be evenly distributed throughout the thickness of media, or it may be fiber heavy on one side and particulate heavy on the other. This controllable gradient structure allows one to control filter media permeability an adsorptive rate therefore present to filter designer a flexibility that is previously unavailable.
Still another objective of the present invention is based on the aerodynamic process to produce to a chemical filter media with minimum channeling effect. A channeling effect is said to be taking place when contaminant is able to find its way from upstream to downstream through largest spaces between adjacent particulates (air channel), without being adsorbed by particulate. To minimize channeling effect, air channels must be consistently small and tortuous, and must allow airflow pass through at closest possible distance from particulates with longest possible residence time.
Because the aerodynamic interlacing technology integrates particulate and short cut fibers simultaneously, instead of depositing particulate into a preformed substrate, openings formed between adjacent particulates are consistently small. More over, openings between adjacent particulates are advantageously filled with a controlled amount of fibers (a fiber aggregate) to divert airflow passing through the media.
This improved micro-structure is further enhanced by a thermo contraction process immediately follows. When heated, sheath material of bi-component fibers is melted to bond to one another. This bonding immobilizes functional particulate and provides tensile strength to resulting filter media. This thermo process also facilitates a controlled contraction to further reduce size of air channels. Furthermore, as the media is quenched, a slight gap will form between fiber aggregate and particulates, this is because materials are likely to have stronger bonding to more compatible materials (fibers to fibers), hence pulled away from lesser compatible materials (fibers to particulate). As a result, fiber aggregates become an effective diverting mechanism that forces airflow to pass through air channels at a very close distance from particulate surface.